High power optical fiber lasers are becoming of increasing interest as a consequence of the efficiency, lower cost, and availability of arrays of diode pump lasers. Such arrays of diode pump lasers can have an output power approaching 100 W or greater, and serve as an ideal pump source for optical fiber lasers.
The purpose of the optical fiber laser is two fold. First, the output from a diode array is highly multi-mode, and thus, there is a limiting factor to the brightness. There are many situations in which the beam quality of a single mode fiber with less power is more desirable than a higher powered multi-mode array. These applications can include materials processing, surgery, sources for high power communications between satellites, as well as applications in terrestrial communications systems. A second reason is that the fiber laser can transform the wavelength of the diode array to a different wavelength regime, preferably one that is not currently available to high power diodes. An output emission in the "eye safe" region beyond 1.5 microns would fall into this category. Also, and with specific reference to surgical applications, a fiber laser operating at 1.95 microns has two orders of magnitude higher water absorption, and thus a higher interactivity with tissue, than does a diode array operating at either 800 nm or 980 nm.
When the energy from a diode array is focused into a fiber laser, if the laser is single mode, there will be, at best, inefficient coupling into the fiber core. Much of the energy will be present in the form of skew, or helical rays that merely circle the axis of the fiber where the core is located. If, however, this symmetry is broken by placing the core off-center, or if a slab geometry with one axis of the slab longer than the other, it will be possible for these rays to intersect the core as they travel along the fiber. In the case of the slab geometry, an outer coating of a low index of refraction plastic can cover the slab. The combination then functions as a multi-mode structure that guides the high energy of the pump light from the diode array. Also, some diode arrays are configured as rectangles, which provides a suitable geometry for coupling into the multi-mode rectangle in the fiber. A known type of laser constructed in accordance with this approach may be referred to as a "double clad" optical fiber laser.
One of the disadvantages of present double-clad optical fibers is that the outer coating is a polymer. While this coating material has been shown to be suitable to permit single mode operation (CW) at a level of 50 W in a ytterbium fiber laser, it is clear that as power demands increase a polymeric coating will not be adequate. This is true at least for the reason that any interaction of the high power laser diode pump source with the polymeric cladding or coating layer will destroy the laser.
Also, the lowest index of refraction polymer that is presently available is approximately 1.35. This implies that the numerical aperture of the fiber is limited to less than 0.5. The numerical aperture of the fiber laser is a measure of the acceptance angle for the pump light, and the higher the number, the easier it is to couple the high energy pump source into the fiber. By example, space communications applications require as high a numerical aperture as possible and, for reasons of long term reliability, an all-glass fiber laser.
Diode bar arrays are now commercially available, and can be arranged to produce power levels in the many tens of Watts. The power is delivered through multi-mode fibers, or arrays of fibers bundled together. However, a fiber having a low numerical aperture will not efficiently accept the radiation of these devices in such a manner that single mode laser radiation will result, as would occur by having the pump radiation from the diode array serve as the energy source to create an inversion in a rare earth doped optical fiber laser. Since single mode operation is desired for high brightness, with a core typically 5-10 microns in diameter, it is not possible to focus the light from the fiberized output of the diode array into the single mode core. The brightness theorem specifies that the numerical aperture of the fibers coming from the diode source, times the fiber area, must be a constant. Thus, the high intensity light from the fiberized output of the diode array cannot be focused into the core of a single mode fiber.
In order to circumvent this restriction, and as was alluded to above, it known that a fiber preform can be ground into a rectangle with the core in the center. The preform is pulled with a low index polymer as a coating, which serves to confine light focused into the rectangular region. As the light proceeds down this rectangular section of the fiber, it bounces back and forth, and is absorbed by the core of the fiber. This creates an inversion in the core, which is single mode, and is an indirect technique for avoiding the restriction of the brightness theorem which states, as noted above, that the product of area times numerical aperture (for the source and fiber receiving radiation) must remain constant.
A distinct disadvantage of this type of cladding pumped fiber is that the outer cladding which confines the light in the rectangular area is a soft plastic. If any of the high intensity light interacts with this material, it will instantly be degraded. Also, in space applications, or if one desires to deposit or sputter an anti-reflecting coating on the fiber end, the soft plastic material will significantly outgas.
Nevertheless, these fibers are playing an increasingly important role in telecommunications, space applications, and materials processing.
In light of the above discussion, a consideration is now made of an alternate approach that has led to an all glass, higher numerical aperture fiber. First, however, reference is made to early work done at Bell Laboratories on optical fibers by Kaiser et al. In this work, which predated any of the now commonly used techniques for the fabrication of optical fibers, it was realized that an index difference was necessary to obtain guiding in a glass fiber. The solution that was arrived at, since at that time there were no suitable techniques for preform fabrication, was that of an "airplane" fiber. In this fiber, a quartz slide was placed in a quartz tube, and a rod was placed along the quartz slide. Effectively, the numerical aperture of this device was high, because the quartz core was surrounded by air. After a fiber was drawn the losses were such that transmission over a hundred meters was obtained.